1. Field of the Invention
This invention relates to the production of aluminium alloy in strip form.
2. Prior Art
In a continuous process line for producing an aluminium alloy in strip form, cold rolling is often used to obtain a strip with accurate dimensions and a good surface. Cold rolling leaves the material in a work-hardened state in which it tends to be strong but not very ductile. To restore the ductility, the strip is annealed by maintaining it for a suitable time at a temperature in the range of 200.degree.-450.degree. C. Another method of obtaining high strengths in aluminium alloy strip is precipitation hardening. This precipitation hardening treatment hardens the strip by causing an extremely fine precipitate dispersion to form in the matrix. The heat treatment required to produce this dispersion consists of solution treatment in which any pre-existing precipitates are dissolved to give a uniform solid solution, the heat treatment being followed by rapid quenching of the material to room temperature so as to retain the solid solution. The quenching is followed by ageing, which can take place at room temperature and/or temperatures in the range of 100.degree. to 200.degree. C. during which the solid solution breaks down to give the fine precipitate dispersion.
On a continuous heat treatment line, there is a possibility of temperature variations across the width of the strip. In some aluminium alloys where the solution treatment is a sensitive function of the solution treatment time and temperature, such temperature variations across the width of the strip can lead to differential solution treatment across the strip width.
It is known to measure the thermal profile across the width of the strip by a scanning system sensitive to the radiation from the strip.
However radiation sensitive scanning systems have been found to be very sensitive to the emissivity of the surface being scanned. With some alloys, which usually are the same alloys as are very sensitive to the solution treatment conditions, the emissivity changes at high temperatures. This change in emissivity masks the true thermal profile of the strip as detected by radiation sensitive scanning systems.